The complement system plays a central role in the clearance of immune complexes and in immune responses to infectious agents, foreign antigens, virus infected cells and tumor cells. Inappropriate or excessive activation of the complement system can lead to harmful, and even potentially life-threatening consequences due to severe inflammation and resulting tissue destruction. These consequences are clinically manifested in various disorders including septic shock; myocardial, as well as, intestinal ischemia/reperfusion injury; graft rejection; organ failure; nephritis; pathological inflammation; and autoimmune diseases.
The complement system is composed of a group of proteins that are normally present in the serum in an inactive state. Activation of the complement system encompasses mainly three distinct pathways, i.e., the classical, the alternative, and the lectin pathway (V. M. Holers, In Clinical Immunology: Principles and Practice, ed. R. R. Rich, Mosby Press; 1996, 363-391): 1) The classical pathway is a calcium/magnesium-dependent cascade, which is normally activated by the formation of antigen-antibody complexes. It can also be activated in an antibody-independent manner by the binding of C-reactive protein, complexed with ligand, and by many pathogens including gram-negative bacteria. 2) The alternative pathway is a magnesium-dependent cascade which is activated by deposition and activation of C3 on certain susceptible surfaces (e.g. cell wall polysaccharides of yeast and bacteria, and certain biopolymer materials). 3) The lectin pathway involves the initial binding of mannose-binding lectin and the subsequent activation of C2 and C4, which are common to the classical pathway (Matsushita, M. et al., J. Exp. Med. 176: 1497-1502 (1992); Suankratay, C. et al., J. Immunol. 160: 3006-3013 (1998)).
The activation of the complement pathway generates biologically active fragments of complement proteins, e.g. C3a, C4a and C5a anaphylatoxins and C5b-9 membrane attack complexes (MAC), all which mediate inflammatory responses by affecting leukocyte chemotaxis; activating macrophages, neutrophils, platelets, mast cells and endothelial cells; and increasing vascular permeability, cytolysis and tissue injury.
Complement C5a is one of the most potent proinflammatory mediators of the complement system. (The anaphylactic C5a peptide is 100 times more potent, on a molar basis, in eliciting inflammatory responses than C3a.) C5a is the activated form of C5 (190 kD, molecular weight). C5a is present in human serum at approximately 80 μg/ml (Kohler, P. F. et al., J. Immunol. 99: 1211-1216 (1967)). It is composed of two polypeptide chains, α and β, with approximate molecular weights of 115 kD and 75 kD, respectively (Tack, B. F. et al., Biochemistry 18: 1490-1497 (1979)). Biosynthesized as a single-chain promolecule, C5 is enzymatically cleaved into a two-chain structure during processing and secretion. After cleavage, the two chains are held together by at least one disulphide bond as well as noncovalent interactions (Ooi, Y. M. et al., J. Immunol. 124: 2494-2498(1980)).
C5 is cleaved into the C5a and C5b fragments during activation of the complement pathways. The convertase enzymes responsible for C5 activation are multi-subunit complexes of C4b, C2a, and C3b for the classical pathway and of (C3b)2, Bb, and P for the alternative pathway (Goldlust, M. B. et al., J. Immunol. 113: 998-1007 (1974); Schreiber, R. D. et al, Proc. Natl. Acad. Sci. 75: 3948-3952 (1978)). C5 is activated by cleavage at position 74-75 (Arg-Leu) in the α-chain. After activation, the 11.2 kD, 74 amino acid peptide C5a from the amino-terminus portion of the α-chain is released. Both C5a and C3a are potent stimulators of neutrophils and monocytes (Schindler, R. et al., Blood 76: 1631-1638 (1990); Haeffner-Cavaillon, N. et al., J. Immunol. 138: 794-700 (1987); Cavaillon, J. M. et al., Eur. J. Immunol. 20: 253-257 (1990)).
In addition to its anaphylatoxic properties, C5a induces chemotactic migration of neutrophils (Ward, P. A. et al., J. Immunol. 102: 93-99 (1969)), eosinophils (Kay, A. B. et al., Immunol. 24: 969-976 (1973)), basophils (Lett-Brown, M. A. et al., J. Immunol. 117: 246-252 1976)), and monocytes (Snyderman, R. et al., Proc. Soc. Exp. Biol. Med. 138: 387-390 1971)). Both C5a and C5b-9 activate endothelial cells to express adhesion molecules essential for sequestration of activated leukocytes, which mediate tissue inflammation and injury (Foreman, K. E. et al., J. Clin. Invest. 94: 1147-1155 (1994); Foreman, K. E. et al., Inflammation 20: 1-9 (1996); Rollins, S. A. et al., Transplantation 69: 1959-1967 (2000)). C5a also mediates inflammatory reactions by causing smooth muscle contraction, increasing vascular permeability, inducing basophil and mast cell degranulation and inducing release of lysosomal proteases and oxidative free radicals (Gerard, C. et al., Ann. Rev. Immunol. 12: 775-808 (1994)). Furthermore, C5a modulates the hepatic acute-phase gene expression and augments the overall immune response by increasing the production of TNF-α, IL-1-β, IL-6, IL-8, prostaglandins and leukotrienes (Lambris, J. D. et al., In: The Human Complement System in Health and Disease, Volanakis, J. E. ed., Marcel Dekker, New York, pp. 83-118).
The anaphylactic and chemotactic effects of C5a are believed to be mediated through its interaction with the C5a receptor. The human C5a receptor (C5aR) is a 52 kD membrane bound G protein-coupled receptor, and is expressed on neutrophils, monocytes, basophils, eosinophils, hepatocytes, lung smooth muscle and endothelial cells, and renal glomerular tissues (Van-Epps, D. E. et al., J. Immunol. 132: 2862-2867 (1984); Haviland, D. L. et al., J. Immunol. 154:1861-1869 (1995); Wetsel, R. A., Immunol. Leff. 44: 183-187 (1995); Buchner, R. R. et al., J. Immunol. 155: 308-315 (1995); Chenoweth, D. E. et al., Proc. Natl. Acad. Sci. 75: 3943-3947 (1978); Zwirner, J. et al., Mol. Immunol. 36:877-884 (1999)). The ligand-binding site of C5aR is complex and consists of at least two physically separable binding domains. One binds the C5a amino terminus (amino acids 1-20) and disulfide-linked core (amino acids 21-61), while the second binds the C5a carboxy-terminal end (amino acids 62-74) (Wetsel, R. A., Curr. Opin. Immunol. 7: 48-53 (1995)).
C5a plays important roles in inflammation and tissue injury. In cardiopulmonary bypass and hemodialysis, C5a is formed as a result of activation of the alternative complement pathway when human blood makes contact with the artificial surface of the heart-lung machine or kidney dialysis machine (Howard, R. J. et al., Arch. Surg. 123: 1496-1501 (1988); Kirklin, J. K. et al., J. Cardiovasc. Surg. 86: 845-857 (1983); Craddock, P. R. et al., N. Engl. J. Med. 296: 769-774 (1977)). C5a causes increased capillary permeability and edema, bronchoconstriction, pulmonary vasoconstriction, leukocyte and platelet activation and infiltration to tissues, in particular the lung (Czermak, B. J. et al., J. Leukoc. Biol. 64: 40-48 (1998)). Administration of an anti-C5a monoclonal antibody was shown to reduce cardiopulmonary bypass and cardioplegia-induced coronary endothelial dysfunction (Tofukuji, M. et al., J. Thorac. Cardiovasc. Surg. 116: 1060-1068 (1998)).
C5a is also involved in acute respiratory distress syndrome (ARDS), Chronic Obstructive Pulmonary Disorder (COPD) and multiple organ failure (MOF) (Hack, C. E. et al., Am. J. Med. 1989: 86: 20-26; Hammerschmidt D E et al. Lancet 1980; 1: 947-949; Heideman M. et al. J. Trauma 1984; 4: 1038-1043; Marc, M M, et al., Am. J. Respir. Cell and Mol. Biol., 2004: 31: 216-219). C5a augments monocyte production of two important pro-inflammatory cytokines, TNF-α and IL-1. C5a has also been shown to play an important role in the development of tissue injury, and particularly pulmonary injury, in animal models of septic shock (Smedegard G et al. Am. J. Pathol. 1989; 135: 489-497; Markus, S., et al., FASEB Journal (2001), 15: 568-570). In sepsis models using rats, pigs and non-human primates, anti-C5a antibodies administered to the animals before treatment with endotoxin or E. coli resulted in decreased tissue injury, as well as decreased production of IL-6 (Smedegard, G. et al., Am. J. Pathol. 135: 489-497 (1989); Hopken, U. et al., Eur. J. Immunol. 26: 1103-1109 (1996); Stevens, J. H. et al., J. Clin. Invest. 77: 1812-1816 (1986)). More importantly, blockade or C5a with anti-C5a polyclonal antibodies has been shown to significantly improve survival rates in a caecal ligation/puncture model of sepsis in rats (Czermak, B. J. et al., Nat. Med. 5: 788-792 (1999)). This model share many aspects of the clinical manifestation of sepsis in humans. (Parker, S. J. et al., Br. J. Surg. 88: 22-30 (2001)). In the same sepsis model, anti-C5a antibodies were shown to inhibit apoptosis of thymocytes (Guo, R. F. et al., J. Clin. Invest. 106: 1271-1280 (2000)) and prevent MOF (Huber-Lang, M. et al., J. Immunol. 166: 1193-1199 (2001)). Anti-C5a antibodies were also protective in a cobra venom factor model of lung injury in rats, and in immune complex-induced lung injury (Mulligan, M. S. et al. J. Clin. Invest. 98: 503-512 (1996)). The importance of C5a in immune complex-mediated lung injury was later confirmed in mice (Bozic, C. R. et al., Science 26: 1103-1109 (1996)).
C5a is found to be a major mediator in myocardial ischemia-reperfusion injury. Complement depletion reduced myocardial infarct size in mice (Weisman, H. F. et al., Science 249: 146-151 (1990)), and treatment with anti-C5a antibodies reduced injury in a rat model of hindlimb ischemia-reperfusion (Bless, N. M. et al., Am. J. Physiol. 276: L57-L63 (1999)). Reperfusion injury during myocardial infarction was also markedly reduced in pigs that were retreated with a monoclonal anti-C5a IgG (Amsterdam, E. A. et al., Am. J. Physiol. 268:H448-H457 (1995)). A recombinant human C5aR antagonist reduces infarct size in a porcine model of surgical revascularization (Riley, R. D. et al., J. Thorac. Cardiovasc. Surg. 120: 350-358 (2000)).
C5a driven neutrophils also contribute to many bullous diseases (e.g., bullous pemphigoid, pemphigus vulgaris and pemphigus foliaceus). These are chronic and recurring inflammatory disorders clinically characterized by sterile blisters that appear in the sub-epidermal space of the skin and mucosa. While autoantibodies to keratinocytes located at the cutaneous basement membranes are believed to underlie the detachment of epidermal basal keratinocytes from the underlying basement membrane, blisters are also characterized by accumulation of neutrophils in both the upper dermal layers and within the blister cavities. In experimental models a reduction of neutrophils or absence of complement (total or C5-selective) can inhibit formation of sub-epidermal blisters, even in the presence of high autoantibody titers.
Complement levels are elevated in patients with rheumatoid arthritis (Jose, P. J. et al., Ann. Rheum. Dis. 49: 747-752 (1990); Grant, E. P., et al., J. of Exp. Med., 196(11): 1461-1471, (2002)), lupus nephritis (Bao, L., et al., Eur. J. of Immunol., 35(8), 2496-2506, (2005)) and systemic lupus erythematosus (SLE) (Porcel, J. M. et al., Clin. Immunol. Immunopathol. 74: 283-288 (1995)). C5a levels correlate with the severity of the disease state. Collagen-induced arthritis in mice and rats resembles the rheumatoid arthritic disease in human. Mice deficient in the C5a receptor demonstrated a complete protection from arthritis induced by injection of monoclonal anti-collagen Abs (Banda, N. K., et al., J. of Immunol., 2003, 171: 2109-2115). Therefore, inhibition of C5a and/or C5a receptor (C5aR) could be useful in treating these chronic diseases.
The complement system is believed to be activated in patients with inflammatory bowel disease (IBD) and is thought to play a role in the disease pathogenesis. Activated complement products were found at the luminal face of surface epithelial cells, as well as in the muscularis mucosa and submucosal blood vessels in IBD patients (Woodruff, T. M., et al., J of Immunol., 2003, 171: 5514-5520).
C5aR expression is upregulated on reactive astrocytes, microglia, and endothelial cells in an inflamed human central nervous system (Gasque, P. et al., Am. J. Pathol. 150: 31-41 (1997)). C5a might be involved in neurodegenerative diseases, such as Alzheimer disease (Mukherjee, P. et al., J. Neuroimmunol. 105: 124-130 (2000); O'Barr, S. et al., J. Neuroimmunol. (2000) 105: 87-94; Farkas, I., et al. J. Immunol. (2003) 170:5764-5771), Parkinson's disease, Pick disease and transmissible spongiform encephalopathies. Activation of neuronal C5aR may induce apoptosis (Farkas I et al. J. Physiol. 1998; 507: 679-687). Therefore, inhibition of C5a and/or C5aR could also be useful in treating neurodegenerative diseases.
There is some evidence that C5a production worsens inflammation associated with atopic dermatitis (Neuber, K., et al., Immunology 73:83-87, (1991)), and chronic urticaria (Kaplan, A. P., J. Allergy Clin. Immunol. 114; 465-474, (2004).
Psoriasis is now known to be a T cell-mediated disease (Gottlieb, E. L. et al., Nat. Med. 1: 442-447 (1995)). However, neutrophils and mast cells may also be involved in the pathogenesis of the disease (Terui, T. et al., Exp. Dermatol. 9: 1-10; 2000); Werfel, T. et al., Arch. Dermatol. Res. 289: 83-86 (1997)). Neutrophil accumulation under the stratum corneum is observed in the highly inflamed areas of psoriatic plaques, and psoriatic lesion (scale) extracts contain highly elevated levels of C5a and exhibit potent chemotactic activity towards neutrophils, an effect that can be inhibited by addition of a C5a antibody. T cells and neutrophils are chemo-attracted by C5a (Nataf, S. et al., J. Immunol. 162: 4018-4023 (1999); Tsuji, R. F. et al., J. Immunol. 165: 1588-1598 (2000); Cavaillon, J. M. et al., Eur. J. Immunol. 20: 253-257 (1990)). Additionally expression of C5aR has been demonstrated in plasmacytoid dendritic cells (pDC) isolated from lesions of cutaneous lupus erythematous and these cells were shown to display chemotactic behavior towards C5a, suggesting that blockade of C5aR on pDC might be efficacious in reducing pDC infiltration into inflamed skin in both SLE and psoriasis. Therefore C5a could be an important therapeutic target for treatment of psoriasis.
Immunoglobulin G-containing immune complexes (IC) contribute to the pathophysiology in a number of autoimmune diseases, such as systemic lupus erthyematosus, rheumatoid arthritis, Sjogren's disease, Goodpasture's syndrome, and hypersensitivity pneumonitis (Madaio, M. P., Semin. Nephrol. 19: 48-56 (1999); Korganow, A. S. et al., Immunity 10: 451-459 (1999); Bolten, W. K., Kidney Int. 50: 1754-1760 (1996); Ando, M. et al., Curr. Opin. Pulm. Med. 3: 391-399 (1997)). These diseases are highly heterogeneous and generally affect one or more of the following organs: skin, blood vessels, joints, kidneys, heart, lungs, nervous system and liver (including cirrhosis and liver fibrosis). The classical animal model for the inflammatory response in these IC diseases is the Arthus reaction, which features the infiltration of polymorphonuclear cells, hemorrhage, and plasma exudation (Arthus, M., C.R. Soc. Biol. 55: 817-824 (1903)). Recent studies show that C5aR deficient mice are protected from tissue injury induced by IC (Kohl, J. et al., Mol. Immunol. 36: 893-903 (1999); Baumann, U. et al., J. Immunol. 164: 1065-1070 (2000)). The results are consistent with the observation that a small peptidic anti-C5aR antagonist inhibits the inflammatory response caused by IC deposition (Strachan, A. J. et al., J. Immunol. 164: 6560-6565 (2000)). Together with its receptor, C5a plays an important role in the pathogenesis of IC diseases. Inhibitors of C5a and C5aR could be useful to treat these diseases.